1. Technical Field
The invention relates generally to signal detection and channel parameter estimation for multiple subcarrier signals. More particularly, the invention relates to a method for multicarrier signal detection and parameter estimation in mobile radio communication channels.
2. Description of the Prior Art
In multiple subcarrier signaling, the entire bandwidth is divided into several consecutive subbands. The subcarrier frequency located at the center of each subband is modulated by a relatively low speed digital signal. The channel delay spread, .tau., is much smaller than the symbol duration, T. T is defined as T=1/f.sub.s, where f.sub.s is the symbol rate.
The total symbol rate of the subcarrier system is obtained by multiplying the symbol rate of each subcarrier by the number of subcarriers. Thus, if the symbol rate of each subcarrier is f.sub.s and there are M subcarriers, the total symbol rate is Mf.sub.s.
The total symbol rate of a multiple subcarrier system may require an entire bandwidth larger than or equivalent to the channel coherence bandwidth.
However, each subcarrier's bandwidth is small enough to prevent the received signal from being damaged by inter-symbol interference (ISI). Hence, multiple subcarrier signaling is known in the prior art as an effective method for reducing the effects of fading frequency selectivity encountered in mobile communications channels. In such systems, the need for channel equalizers, which might be required for a single carrier signaling scheme with the same (total) symbol rate (Mf.sub.s), is eliminated.
FIG. 1 is a graphical representation of a bandwidth 10 divided into multiple subcarriers 12, 14, 16, 18. Frequency-selective fading 20 may cause the degradation of the signal over the entirety of the bandwidth. However, because each individual subcarrier has such a small subband width 21, the fading can be approximated as frequency-flat fading for each of the subcarriers.
Recently, pilot symbol-assisted modulation (PSAM) has been proposed for mobile communication applications. In PSAM, the fading complex envelope is estimated using pilot symbols periodically embedded in the information symbol sequence to be transmitted. FIG. 2 is an example of a symbol sequence frame format 22 according to the prior art. Pilot symbols 24 are periodically embedded in the information symbol 26 sequence. For coherent detection, the complex conjugate of the fading envelope estimate is multiplied by the received signal sample.
It is known to use interpolation techniques for the fading estimation, and apply it to the multilevel quadrature amplitude modulation (QAM) signal transmission over Rayleigh fading channels. See, for example, S. Sampei and T. Sunaga, Rayleigh Fading Compensation for 16QAM in Land Mobile Radio Communications, IEEE Trans. VT., vol. VT-42, pp. 137-147. (May 1993); and T. Sunaga and S. Sampei, Performance of Multi-Level OAM with Post-Detection Maximal Ratio Combining Space Diversity for Digital Land-Mobile Radio Communications, IEEE Trans. VT., vol. VT-42, pp. 294-301. (August 1993).
PSAM signal detection has also been combined with decoding of trellis codes. See, for example, M. L. Moher and J. H. Lodge, TCMP--A Modulation and Coding Strategy for Rician Fading Channels, IEEE JSAC., vol. SAC-7, pp. 1347-1355. (December 1989); and A. N. D'Andrea, A. Diglio and U. Menglai, Symbol-Aided Channel Estimation with Nonselective Fading Channels, IEEE Trans. VT, Vol. VT-44, pp. 41-49. (1995).
Recently, the performance limit of the pilot-assisted coherent signal detection has been analyzed where a Wiener filter is used to minimize the variance of the estimation error within the class of linear filters. See, for example, J. K. Cavers, An Analysis of Pilot Symbol-Assisted Modulation for Rayleigh Fading Channels, IEEE Trans. VT., Vol. VT-40, pp. 686-693. (1991).
Unlike the case of single carrier signaling, information about the fading complex envelope can be more frequently extracted by locating the pilot symbols at different timings for some of the subcarriers when PSAM is used in subcarrier signaling. FIG. 3 shows an example of a multiple subcarrier PSAM format. The pilot symbols 24 are imbedded at different timings 36, 37 within the information symbol 26 sequence for some of the subcarriers 28, 30, 32, 34.
With PSAM, the pilot symbols of each of the subcarriers are transmitted at a constant frequency. This suggests the use of pilot symbols for more precise fading estimation than the single carrier's case while maintaining a constant, overall spectrum efficiency (=per-subcarrier information symbol rate /f.sub.s).
With multiple subcarrier signaling, each of the subcarriers suffers from almost frequency-flat fading because .tau.fs&lt;&lt;1, where .tau. is the channel delay spread. The fading complex envelopes with the M subcarriers are different from each other, but correlate closely. Therefore, this scheme uses neither the frequent pilot symbol reception nor the fading correlation among the subcarriers if fading estimation for coherent detection takes place subcarrier-by-subcarrier.
FIG. 4 is a graph illustrating subcarrier-by-subcarrier detection according to the prior art. Using this method, the power spectrum of the fading envelope 38 at time t is constant for all subcarrier frequencies. The prior art method does not efficiently use fading correlation. Even under frequency-flat fading, fading estimates obtained through signal detections for subcarriers 32 and 34 are not used for subcarrier 28 and 30 signal detections.
One significant drawback of the PSAM signal detection method is that it directly estimates the fading complex envelope. FIG. 5 is a graph illustrating frequency-selective fading according to the prior art. Under these conditions, the power spectrum of the fading envelope 38 at time t differs for varying subcarrier frequencies. As with frequency-flat fading, fading estimates obtained through subcarrier 32 and 34 signal detections cannot be used for subcarrier 28 and 30 signal detections.
If, however, parameters common to all subcarriers and related to the generation process of fading frequency selectivity are estimated, knowledge about fading obtained from other subcarriers can be used for the signal detection of the subcarrier of interest. Thus, as is shown in the graph of FIG. 6, parameter estimates obtained through subcarrier 32 and 24 signal detections can be used for subcarrier 28 and 30 signal detections.
It would be an advantage to provide a joint signal detection and channel parameter estimation scheme for the coherent detection of multiple subcarrier PSAM signals. It would be a further advantage if such a system made effective use of the pilot symbols received by different subcarriers, not necessarily the pilot symbol of interest.